REVIEW ARTICLE https://doi.org/10.1038/s41550-020-1051-1 Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies Aigen Li Polycyclic aromatic hydrocarbon (PAH) molecules are abundant and widespread throughout the Universe, as revealed by their distinctive set of emission bands at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 μm, which are characteristic of their vibrational modes. They are ubiquitously seen in a wide variety of astrophysical regions, ranging from planet-forming disks around young stars to the interstellar medium of the Milky Way and other galaxies out to high redshifts at z ≳ 4. PAHs profoundly influence the thermal budget and chemistry of the interstellar medium by dominating the photoelectric heating of the gas and controlling the ionization balance. Here I review the current state of knowledge of the astrophysics of PAHs, focusing on their observational characteristics obtained from the Spitzer Space Telescope and their diagnostic power for probing the local physical and chemi- cal conditions and processes. Special attention is paid to the spectral properties of PAHs and their variations revealed by the Infrared Spectrograph onboard Spitzer across a much broader range of extragalactic environments (for example, distant galax- ies, early-type galaxies, galactic halos, active galactic nuclei and low-metallicity galaxies) than was previously possible with the Infrared Space Observatory or any other telescope facilities. Also highlighted is the relation between the PAH abundance and the galaxy metallicity established for the first time by Spitzer. n the early 1970s, a new chapter in astrochemistry was opened by some of the longstanding unexplained interstellar phenomena (for Gillett et al.1 who, on the basis of ground observations, detected example, the 2,175 Å extinction bump9,16,19, the diffuse interstellar three prominent emission bands peaking at 8.6, 11.3 and 12.7 μm bands20, the blue and extended red photoluminescence emission21 I 22,23 in the 8–14 μm spectra of two planetary nebulae, NGC 7027 and and the ‘anomalous microwave emission’ ). PAHs profoundly BD+30°3639. Two years later, Merrill et al.2 reported the detection influence the thermal budget and chemistry of the ISM. They domi- of a broad emission band at 3.3 μm, again in NGC 7027. Also around nate the heating of the gas in the diffuse ISM as well as the surface that time, airborne observations became possible. This led to the layers of protoplanetary disks by providing photoelectrons24–26. As detection of two additional, ground-inaccessible intense emission an important sink for electrons, PAHs dominate the ionization bal- bands at 6.2 and 7.7 μm in NGC 70273 and M82, an external gal- ance in molecular clouds and hence they influence the ion–mol- axy4, with the Kuiper Airborne Observatory (KAO). Subsequently, ecule chemistry and the ambipolar diffusion process that sets the all these features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 μm were found to stage for star formation27. be widespread throughout the Universe and closely related to form In this Review, I provide an overview of the current state of a family, exhibiting an overall similar spectral profile among the dif- knowledge of the astrophysics of PAHs, with details on their obser- ferent sources (Fig. 1). vational characteristics obtained from the Spitzer Space Telescope. Although the exact nature of the carriers of these features Special attention is also paid to their diagnostic capabilities to probe remains unknown—because of this, they are collectively known as the local physical and chemical conditions as well as their reac- the ‘unidentified’ infrared emission (UIE) features—the hypothesis tions to different environments. We focus on Spitzer results, but of polycyclic aromatic hydrocarbon (PAH) molecules as the car- the science case often builds on pioneering observations performed riers5,6 has gained widespread acceptance and extreme popularity. before Spitzer with ground-based, airborne and space telescopes, The PAH model attributes the UIE bands to the vibrational modes in particular, the Infrared Space Observatory (ISO). We discuss the of PAHs composed of fused benzene rings of several tens to sev- properties of PAHs in the context of both Spitzer observations and eral hundreds of carbon atoms, with the 3.3 μm band assigned to observations obtained with previous and other contemporary tele- C–H stretching modes, the 6.2 and 7.7 μm bands to C–C stretch- scope facilities. ing modes, the 8.6 μm band to C–H in-plane bending modes, and the 11.3 and 12.7 μm bands to C–H out-of-plane (CHoop) bending PAHs in the pre-Spitzer era modes. The relative strengths of these bands depend not only on the Over the intervening 30 years between the first detection of PAHs size, structure and charging of the PAH molecule but also on the in 19731 and the launch of Spitzer in 2003, numerous ground-based, local physical conditions7–10. airborne and spaceborne observations have substantially promoted It is now well recognized that PAHs are an essential component or even revolutionized our understanding of PAHs in astrophys- of the interstellar medium (ISM) and play an important role in ics. These observations have established that PAHs are a ubiquitous many aspects of astrophysics. They account for ≲15% of the inter- and abundant component of a wide variety of astrophysical regions, stellar carbon9–13 and their emission accounts for up to 20% of the ranging from planetary nebulae, protoplanetary nebulae, reflection total infrared power of the Milky Way and star-forming galaxies14,15. nebulae, H ii regions, the galactic infrared cirrus and protoplan- Therefore by implication, they must be an important absorber of etary disks around Herbig Ae/Be stars to the ISM of both normal starlight16–18 and are possibly related to or even responsible for and active nearby galaxies15,28. Most notably, Sellgren et al.29 showed Department of Physics and Astronomy, University of Missouri, Columbia, MO, USA. e-mail: [email protected] NATURE ASTRONOMY | VOL 4 | APRIL 2020 | 339–351 | www.nature.com/natureastronomy 339 REVIEW ARTICLE NATURE ASTRONOMY 2.0 a NGC 7023 (reflection nebula) b c 1.0 m μ m μ 7 7. 0.5 ] λ 16.4 F m λ μ /[ λ F 0.2 m λ 7.7 μ m m m μ μ μ 0.1 6.2 8.6 1.3 17 μm 1 12.7 complex M17 (PDR) Orion (PDR) 0.05 2.0 d ef 1.0 m μ m m μ μ 5.7 7 m m 7. 0.5 ] μ μ λ m 5.25 m F m μ m μ λ μ μ /[ λ 14.2 13.5 F 0.2 10.5 λ 7.25 6.85 6.85 0.1 NGC 7027 (planetary nebula) HD 34700 (T Tauri disk) HD 169142 (Herbig Ae/Be disk) 0.05 2.0 g h DCId 300.2-16.9 i NGC 5194 (Seyfert) PAH model (λI /[λI ]7.7 m) (high-latitude cloud) λ λ μ 1.0 m μ 7 0.5 7. ] λ F ] λ /[ III λ ] 0.2 ] F [S S(1) II λ III 2 H II] [Ne 0.1 [Ne 1 S(3) [Ar 5 2 10 H 0.05 5710 15 5710 15 5710 15 λ (μm) λ (μm) λ (μm) Fig. 1 | Observed and model-predicted 5–20 μm PAH spectra. a, Reflection nebula NGC 702356. b, M17 photodissociation region (PDR)171. c, Orion Bar PDR172. d, Planetary nebula NGC 7027173. e, T Tauri disk HD 34700174. f, Herbig Ae/Be disk HD 169142175. g, Seyfert grand-design spiral galaxy NGC 519414. h, Translucent high-galactic-latitude cloud DCld 300.2-16.9176. i, Model emission calculated for PAHs illuminated by the local interstellar radiation field177 (that is, U = 1; black line) or a much more intense radiation field (that is, U = 105; red line)10. The major PAH bands are labelled in a. Some of the weak, secondary PAH bands (superimposed by sharp gas lines) are labelled in d. The 6.85 and 7.25 μm aliphatic C–H deformation bands are labelled in e and f for protoplanetary disks. The sharp gaseous emission lines are labelled in g. Fλ is the observed flux at wavelength λ. U is the starlight intensity measured in 177 10 the unit of the local interstellar radiation field . Iλ is the model emission intensity . that in three reflection nebulae (NGC 7023, NGC 2023 and NGC In the 1970s and 1980s, the 6.2 and 7.7 μm bands were only 2068) the 3.3 μm feature profile shows very little variation with dis- accessible from the KAO. KAO observations had already revealed tance from the central star, revealing the emission mechanism of substantial variations in the PAH feature profiles among different the PAH bands as due to the infrared fluorescence from molecule- types of sources. Typically, harsh environments, such as planetary sized species vibrationally excited by individual ultraviolet/visible nebulae, reflection nebulae and H ii regions, in which the dust has photons7,30,31. been heavily processed, show ‘normal’-looking PAH spectra. The Owing in large part to the fact that the 3 μm region and the 8–14 7.7 and 8.6 μm bands of these sources are well separated. In con- μm region are accessible to ground-based telescopes, in the 1980s trast, some benign protoplanetary nebulae, in which the dust is 36 and 1990s, the C–H stretching bands at 3.3 μm and the CHoop bend- relatively fresh, exhibit a broad 8 μm complex . The CHoop bending ing bands at 11.3 μm were the subject of extensive scrutiny.